Research in my lab focuses on the organization and development of the mammalian visual system. This proposal describes studies which investigate neuronal structure and its development in the dorsal lateral geniculate nucleus of the cat. Many of the experiments rely on the ability to fill neurons with horseradish peroxidase (HRP) so that axons and dendrites can be studied with the light microscope and the electron microscope. Other studies take advantage of recent findings that a specific population of small neurons in the thalamus can be selectively stained with an antibody to glutamic acid decarboxylase (GAD), the major enzyme for the biosynthesis of gamma-aminobutyric acid (GABA), a known inhibitory neurotransmitter. These may be local circuit interneurons. Studies using adult cats will examine normal neuronal structure. Lateral geniculate neurons will be filled with HRP and their dendritic structure studied with the light and electron microscopes. These studies will focus on cells that give rise to dendrites that cross laminar borders ("translaminar dendrites"). These cells are of special interest because they may be innervated binocularly. The studies of postnatal development will follow two lines. In one, retinogeniculate axons, corticogeniculate axons and lateral geniculate neurons will be filled with HRP in an age-sequenced series of animals. Both of these types of axon contact the same lateral geniculate neurons in adults but the patterns of synaptic contacts are quite different. Thus, a main objective of this study is to look for differences in morphological development that might correlate with, and account for the structural differences seen in adults. Another objective of this study is to trace the development of translaminar dendrites, especially with regard to their synaptic interactions with retinogeniculate axons. The results should provide considerable new information about the development of synaptic specificity in the nucleus. In a second series of experiments the postnatal development of GAD immunoreactivity will be traced. The goal of the study is to determine if the development of GAD immunoreactivity correlates with the development of inhibitory mechanisms that are known to arise postnatally in the lateral geniculate nucleus.